Examples of the overcharge-preventive mechanism for battery pack comprising a lithium ion secondary battery (hereinafter simply referred to as “battery”) having a high energy density include those equipped with a heat-sensitive current blocking element for sensing the temperature rise of the battery to restrict the current flow.
The heat-sensitive current blocking element is normally mounted interposed between lead wires provided on the outer surface of the main body of the battery. When overcharging or the like occurs with abnormal heat generation in the battery, the heat-sensitive current blocking element senses the heat to block or reduce the current flow in the lead wires, making it possible to prevent thermal runaway of the battery.
Specific examples of such a heat-sensitive current blocking element include PTC (Positive Temperature Coefficient) element, thermal fuse, and bimetal device. The PTC element comprises a resistor having a positive temperature coefficient such that the resistivity thereof rises as the temperature rises. When the temperature reaches to the predetermined level, the PTC element shows a sudden rise to restrict the current flow. A thermal fuse comprises a lead wire made of a low melting alloy incorporated therein which melts at a relatively low temperature. When the temperature reaches to the predetermined level, this lead wire fuses to block the current flow. A bimetal device comprises a laminate of two sheets of metal plate having different expansion coefficients and contact point. When the temperature reaches to the predetermined level, the two sheets of metal plate move away from contact point to block the current flow.
In order to assure the operation of the heat-sensitive current blocking element, it is necessary that the rise of the temperature in the battery be transferred to the heat-sensitive current element rapidly and surely. To this end, the heat-sensitive current blocking element is normally mounted in close contact with the outer surface of the main body of battery.
However, when the battery pack undergoes an impact developed by vibration or caused when it drops, the heat-sensitive current blocking element can be separated from the outer surface of the main body of battery. Under these circumstances, when the temperature in the battery rises, the heat-sensitive current blocking element cannot rapidly sense the heat, causing a delay in the operation of restricting current flow. When this delay increases, the temperature in the battery rises excessively before the current flow is restricted, making it likely that the battery can undergo thermal runaway.